Corresponding methods for ascertaining and monitoring fill level in a container are utilized frequently in measuring devices of automation- and process-control-technology. Produced and sold by the assignee, for example, are measuring devices under the mark Levelflex, which work according to a travel-time measuring method and serve to determine and/or monitor the fill-level of a medium in a container. In the method of guided microwaves, or the time-domain reflectometry (TDR) measuring technique, a high-frequency measuring signal, such as e.g. a high-frequency pulse, is transmitted along a waveguide, and, at a jump in the dielectric constant of the medium surrounding the waveguide, reflected back, at least partially, as an echo signal. On the basis of the time period, or travel time, between the transmitting of the high-frequency pulses and the receipt of the reflected, echo signals, taking into consideration the propagation velocity of the high-frequency measuring signal, the distance from the measuring device to the surface of the medium can be ascertained. Given the geometry of the container interior, then the fill-level of the medium is ascertained as a relative or absolute quantity. An advantage of time-domain reflectometry is its almost complete independence of product characteristics such as density, dielectric constant or conductivity, of process conditions such as unsettled surfaces or foam formation, as well as its independence of properties of the container, such as form, size or installed objects.
The travel-time measuring method can be divided essentially into two methods of ascertainment: The first method rests on a measurement of the travel-time, which a pulse sequence, modulated signal requires for the traveled path; while a second widely distributed method of ascertainment rests on determining the frequency difference between the currently transmitted, continuously frequency-modulated, high-frequency signal and the received, reflected, high-frequency signal (FMCW—Frequency-Modulated Continuous Wave). In general, no limitation to a certain method of ascertainment is intended in the following explanations.
In today's state of the art, there are various approaches for determining exact position of the fill-level-representing, wanted echo signal in the ascertained echo curve or the digitized envelope curve. An exact determining of the measured position of the fill level in the echo curve depends, however, on the accuracy of measurement reachable with this echo measuring principle under the given measuring conditions. One approach for determining the fill-level is, in such case, to select that echo signal, which has the largest amplitude, as the wanted echo signal in the echo curve. Under real measuring conditions, it is, however, often not possible to determine the exact fill-level in the container, since, for example, the base point cannot be determined exactly and/or the travel time of the measuring signal varies along the waveguide.
In a fill-level measuring device, most often, a reference reflection is taken into consideration for ascertaining the base point of the travel time measurement. Suited as reflection locations are local changes on the waveguide or in the vicinity of the waveguide, such as e.g. changes of the cross section of metal or dielectric structures in the form of thickening, necking or other irregularity, changes of the dielectric properties at locations where the electrical field is different from zero, changes of the magnetic properties at locations where the magnetic field is different from zero, and/or changes of the conductivity at locations where the electrical current density is different from zero.
In the case of a TDR-measuring method utilizing a waveguide, either the transmission pulse itself is used as base point, such being illustrated in U.S. Pat. No. 3,424,002 and in U.S. Pat. No. 3,995,212, or a disturbance reflection within the measurement-inactive part of the sensor is used, such as disclosed in EP 0 780 664 A2. A further opportunity for establishing the base point of the TDR-measuring method utilizing a waveguide is to use the disturbance location at the transition from the measurement-inactive area to the measurement-active area of the measuring probe or waveguide, the so called “fiducial”, as base point, such as described in U.S. Pat. No. 3,474,337 and in DE 44 04 745 A1. Moreover, a further method is disclosed in U.S. Pat. No. 5,345,471, where a defined disturbance location at the transition from the measurement-inactive area to the measurement-active area of the waveguide establishes this base point using a switched resistive component between the measuring probe and the flange or the outer conductor.
A further application of disturbance locations on waveguides, or measuring probes, is the calibration of the travel time of the measuring signals along the waveguide of a fill-level measuring device. These disturbance locations are, most often, brought about by a changing of the measuring-probe geometry. For calibrating a travel-time measurement utilizing a TDR-fill-level measuring device, a number of disturbance locations on the waveguide can be applied. These disturbance locations are, for example, located in the measurement-active area of the sensor, such as especially shown in U.S. Pat. No. 3,398,578, EP 0 534 654 B1 and U.S. Pat. No. 3,474,337 involving separators on a coaxial measuring probe as reference markers. As a further opportunity for calibrating a travel-time measurement utilizing a TDR-fill-level measuring device, two disturbance locations can be used, wherein one is located at the transition from the measurement-inactive region to the active area and one in the measurement-active region, such as mentioned in U.S. Pat. No. 3,474,337 and U.S. Pat. No. 5,249,463, or wherein one is arranged in the measurement-inactive area of the sensor and one in the measurement-active region, such as shown in U.S. Pat. No. 6,867,729 B2.
Another way than by changes in geometry of the waveguide is to produce a disturbance location for calibrating the travel time by, as disclosed in U.S. Pat. No. 5,376,888, providing controllable PIN-diodes as reference markers between the conductors of a multi-conductor probe.
For effective diagnosis and calibration of a TDR-fill-level measuring device, both the fill-level echo and the base point, as well as also the reflections of the disturbance locations must be exactly and reliably determined. In U.S. Pat. No. 6,867,729 B2, in this connection, via a switchable amplifier, alternately, a disturbance-location echo in the active area of the sensor and a fill-level echo are ascertained. In such case, it is attempted, in the case of changing amplitude of the fill-level echo, as caused, for example, by a changing DK-value of the medium, to perform, nevertheless, an, in some respects, exact determining of the position of the fill-level echo, via a window discriminator, or “constant-fraction-discriminator” (CFD).
Disadvantageous in the case of all these ways of determining fill level is that the base point is dependent on process conditions and measuring conditions, whereby the fill-level of the medium in a container can, due to an undefined base point, or zero point, not be determined exactly.